TOPIC: Concessions cost of good sold

I am starting a three screen independent theatre that will show first and second run films. We want to have a "glorified" concessions stand serving more gourmet snacks and hot items, as well as beer and wine. I'm trying to determine the average monthly costs of food and beverage for my business plan. Is the 33% of sales rule a good one to follow? And is that just 33% of concession sales? This just seems high. Any advice would be appreciated. Thanks!